Gelatin is commonly used in a layered form as a binder in many photographic light-sensitive materials. It has so far been known to harden gelatin using various compounds for the purposes of enhancing water resistance and mechanical strength of the gelatin layer.
For example, known hardeners include aldehyde compounds such as formaldehyde and glutaraldehyde, compounds having a reactive halogen or halogens as described in U.S. Pat. No. 3,288,775, etc., compounds having a reactive, ethylenically unsaturated bond or bonds as described in U.S. Pat. No. 3,642,486, Japanese Patent Publication No. 13563/74, etc., aziridine compounds as described in U.S. Pat. No. 3,017,280, etc., epoxy compounds as described in U.S. Pat. No. 3,091,537, etc., halogen-carboxylaldehydes such as mucochloric acid, etc., dioxanes such as dihydroxydioxane, dichlorodioxane, etc., and inorganic hardeners such as chromium alum, zirconium sulfate, etc.
However, when used in photographic light-sensitive materials, all of these known gelatin hardeners have some drawbacks, such as that only an insufficient hardening effect is provided, that the hardening degree varies with a long period of time, called "post-hardening", due to a slow hardening reaction with gelatin, that detrimental influences (particularly, an increase of fog, a reduction of sensitivity, etc.) are exerted on the properties of photographic light-sensitive materials, that the hardening effect is lost due to other coexisting photographic additives, or that effects of other photographic additives (for example, couplers for color light-sensitive materials) are deteriorated.
As hardeners which exhibit a comparatively rapid hardening reaction with gelatin, and therefore, cause less post-hardening, there are known compounds having a dihydroquinoline skeleton, described in U.S. Pat. No. 4,013,468; compounds having a phosphorus-halogen bond, described in Japanese Patent Application (OPI) No. 113929/83 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"); compounds having an N-sulfonyloximido group, described in U.S. Pat. No. 4,111,962; compounds having two or more N-acyloxyimino groups within them, described in Japanese Patent Publication No. 22089/78; N-carbamoylpyridinium salts described in U.S. Pat. Nos. 3,880,665 and 4,063,952; and 2-sulfonyloxypyridinium salts described in Japanese Patent Application (OPI) No. 110762/81.
These hardeners have the characteristic properties that they cause less post-hardening due to their fast hardening action. However, although they cause a fast hardening reaction with gelatin, at the same time they are subject to the side reaction of being decomposed with water. Therefore, in the common process of preparing a light-sensitive material using a gelatin aqueous solution, the efficiency of the hardener is so low that, in order to obtain a gelatin layer with a desired hardening degree, a large amount of hardener must be used.
Hardeners as described in U.S. Pat. Nos. 3,880,665 and 4,063,952, Japanese Patent Application (OPI) No. 110762/81, etc., are generally believed to undergo a nucleophilic attack with carboxyl or amino groups of gelatin to react therewith, thus hardening gelatin. In preparing a gelatin layer, an aqueous gelatin solution is usually used. Since coexisting water has some nucleophilic properties, it unavoidably tends to react with the hardener to decompose and render the hardener powerless. This tendency is serious with those hardeners which show a fast hardening action. Therefore, in order to harden gelatin rapidly with good efficiency, development of hardeners which react with carboxyl groups, amino groups or the like of gelatin faster than with water, i.e., which well selectively react with gelatin, is of great importance. Hardeners such as N-carbamoylpyridinium salts and 2-sulfonyloxypyridinium salts have the defect that they have such a poor efficiency, which may be attributed to low selectivity as described above, that hardeners which have a higher efficiency, which harden gelatin rapidly, and which have a high water solubility have been eagerly desired to be developed.